In 3rd Generation Partnership Project (3GPP) which is a mobile communication system standardization project, attempts to enhance Long Term Evolution (LTE) in order to satisfy rapidly increasing traffic demands have been made.
As one of such attempts, in addition to a frequency band which is licensed to an operator (licensed band), the use of a frequency band which need not be licensed (unlicensed band) for LTE communication is under review (Non Patent Literature 1).
In the unlicensed band, a listen-before-talk (LBT) procedure is required to prevent interference with a system different from an LTE system (a wireless LAN or the like) or LTE systems of other operators. In the LBT procedure, a base station checks whether or not a frequency channel (carrier) is free on the basis of reception power and uses the frequency channel only when it is checked that it is a free channel (clear channel).